Metallic foams are known to have interesting combinations of physical properties. They offer high stiffness in conjunction with very low specific weight, high gas permeability, and a very high energy absorption ability. Today, these materials are emerging as a new engineering material. Foams can be classified as either open or closed porous. Whereas open foams are mainly used as functional materials such as gas permeability membranes, closed foams find application as structural materials such as energy absorbers or light-weight stiff materials.
However, the broad-use of metallic foams is hindered by the difficulty in producing uniform and consistent foam structures. Specifically, prior manufacturing methods for producing metallic foams result in an undesirably wide distribution of cell and/or pore sizes, which cannot be controlled satisfactorily, and as such limits and degrades the functional and structural characteristics of the metallic foam materials.
The conventional production of metallic foamed structures is generally carried out in the liquid state above the melting temperature of the material, though some solid state methods have also been used. The foaming of ordinary metals is challenging because a foam is an inherently unstable structure. The reason for the imperfect properties of conventional metallic foams comes from the manufacturing process itself. For example, although a pure metal or metal alloy typically consists of a large volume fraction (>50%) of gas bubbles, manufacturing metallic foam from ordinary alloys is very difficult because a desired bubble distribution can not be readily sustained for practical times in their molten state.
Specifically, the time scales for the flotation of bubbles in a foam scales with viscosity of the material. Accordingly, the mechanical properties of these foams drastically degrade with the degree of imperfection caused by the flotation and bursting of bubbles during manufacture. In addition, the low viscosity of most commonly used liquid metals results in a short time scale for manufacture, which makes the processing of metallic foam a delicate process.
In order to remedy these shortcomings, several techniques have been attempted. For example, to reduce the sedimentation flotation process, Ca particles have been added to the liquid metal. However, the addition of Ca degrades the metallic nature of base metal as well as the resultant metallic foam. Alternatively, foaming experiments have been performed under reduced gravity, in space, to reduce the driving force for flotation, however, the cost for manufacturing metallic foams in space is prohibitive.
Accordingly, a need exists for improved methods for manufacturing metallic foams and especially metallic foams of amorphous atomic structure which also can be used for the production of better-controlled foam structures.